1. Field of the Invention
This invention relates to an image recording method for recording an image on a photosensitive material by scanning the photosensitive material with a light beam. This invention particularly relates to a continuous tone image recording method for obtaining a continuous tone image by modulating the intensity of a laser beam emitted by a semiconductor laser and scanning a continuous tone-formable photosensitive material with the laser beam.
2. Description of the Prior Art
Light beam scanning recording apparatuses wherein a light beam is deflected by a light deflector to scan a photosensitive material to record an image thereon have heretofore been widely used. As one of the means for generating the light beam of such apparatuses, a semiconductor laser is used. The semiconductor laser has various advantages over a gas laser or the like in that the semiconductor laser is small, cheap and consumes little power, and can be modulated directly by changing the drive current.
FIG. 2 is a graph showing the relationship between the drive current and the light output of a semiconductor laser. Though the semiconductor laser has various advantages as described above, it has the drawback that the light output characteristics with respect to the drive current sharply change at a point "a" as shown in FIG. 2. Therefore, it is not always possible to use the semiconductor laser recording of a continuous tone image. Specifically, it is not always possible to control the light output across the point "a" at which the light output characteristics change sharply. Further, when intensity modulation is conducted utilizing only the linear characteristic range above the change point "a", the dynamic range of the light output is limited to two orders of magnitude at the most. As is known, with a dynamic range of this order, it is impossible to obtain a continuous tone image of high quality.
Accordingly, various attempts have been made to obtain a continuous tone image by fixing the light output of the semiconductor laser, continuously switching the semiconductor laser on and off to generate scanning beam pulses, and controlling the number of pulses for respective picture elements in accordance with an image density signal, or controlling the pulse width to change the scanning light amount.
However, in the case where pulse number modulation is conducted as described above, in order to ensure density resolution within a range of three orders of magnitude, i.e. scanning light quantity resolution, at a picture element frequency of, for example, 100 kHz, the pulse frequency must be adjusted very high, i.e. to 0.1 GHz (a period of 10 nanoseconds). Though the semiconductor laser can be switched on and off at such a frequency, it is generally impossible for a pulse count circuit for pulse number control or the like to operate at the high frequency. Therefore, the picture element frequency must be decreased markedly below the aforesaid value. Accordingly, when a continuous tone image is recorded by pulse number modulation to ensure high density resolution, the recording speed becomes very low. Also, when pulse width modulation is conducted, in order to satisfy the aforesaid requirement, it is necessary to switch the pulses on and off within at least approximately 10 nanoseconds, and the same problem arises.